Default post body size of express is too small for messages that can be sent

Initializing express without specifying the size of the post body will have the size default to 100Kb but messages can be 256Kb before [payloadNotIncluded](https://docs.commercetools.com/api/projects/subscriptions#ctp:api:type:MessageDeliveryPayload) is used.

I suggest initializing express with a higher post payload body size allowence like: `app.use(bodyParser.json({ limit: '5mb' }));`